The origin and early evolution of plants on land

Abstract

The origin and early diversification of land plants marks an interval of unparalleled innovation in the history of plant life. From a simple plant body consisting of only a few cells, land plants (liverworts, hornworts, mosses and vascular plants) evolved an elaborate two- phase life cycle and an extraordinary array of complex organs and tissue systems. Specialized sexual organs (gametangia), stems with an intricate fluid transport mechanism (vascular tissue), structural tissues (such as wood), epidermal structures for respiratory gas exchange (stomates), leaves and roots of various kinds, diverse spore-bearing organs (sporangia), seeds and the tree habit had all evolved by the end of the Devonian period. These and other innovations led to the initial assembly of plant-dominated terres- trial ecosystems, and had a great effect on the global environment. Early ideas on the origin of land plants were based on living groups, but since the discovery of exceptionally well-preserved fossil plants in the Early Devonian Rhynie Chert, research has focused almost exclusively on the fossil record of vascular plants 1,2 . During the 1970s, syntheses of palaeobotanical and stratigraphic data emphasized the Late Silurian and Devonian periods as the critical interval during which the initial diversification of vascular plants occurred 1,2 , and identified a group of simple fossils (rhyniophytes, such as Cooksonia and Rhynia) as the likely ancestral forms 2 . They